1. Field of the Invention
The present invention relates to an optical pick-up actuator, and more particularly, to a slim type optical pick-up actuator that has a magnetic circuit in a lens holder thereof.
2. Description of the Related Art
Generally, in a device for reading and writing information using an optical disk, an optical pick-up actuator functions to allow laser beam transmitted through an object lens to be accurately located on a track formed on a surface of the disk. With the storage capacity of the optical disk being increased, it is increasingly required to provide a more precision driving control.
In order to provide the more precision driving control, the number of apertures of the object lens is increased. However, the increase of the number of apertures inevitably generates the aberration due to the tilt of the disk. The tilt of the disk deteriorates the playback function and makes it difficult to form fit in the course of writing and information.
Therefore, there is a need for a tilt motion device for correcting the tilt of the disk while performing the tracking motion. There are two typical methods for correcting the tilt: a method for correcting the tilt by moving an overall body of an actuator using a DC motor and a method for correcting the tilt by moving only an optical pick-up motion part (a lens holder) of the actuator.
The former has a problem that it corrects only a disk in a low frequency band and increases an overall size of the optical disk player.
In order to perform the latter, two types of optical pick-up actuators, a moving coil type and a moving magnet type, have been proposed.
However, the moving coil type has at least six wires connected to a driving side to control the tilt motion, complicating the structure. The moving magnet type has a disadvantage of making it difficult to obtain enough sensitivity required for the object lens of the lens holder.
FIGS. 1 through 4 show a 3-axis motion type optical pick-up actuator of related art.
As shown in the drawings, the optical pick-up actuator of related art comprises an object lens 11 for focusing laser beam from an optical disk, tracking and focusing coils 12 and 13 installed on a rear side of the object lens 11, a lens holder 10 provided at a rear surface with tilt magnets 15, and front and rear yokes 22 for fixing magnets for realizing tracking and focusing motions of the tracking and focusing coils 12 and 13.
The optical pick-up actuator further comprises a yoke plate 20 spaced away from the front and rear yokes 22 and provided with a tilt yoke 23, a plurality of wire suspensions 30 disposed on both sides of the lens holder 10 and electrically connected to the tracking and focusing coils 12 and 13, fixing parts 41 for holding the wire suspensions 30 extending therethrough, a yoke insertion member 42 for receiving the tilt yoke 23 disposed between the fixing parts 41, a yoke insertion member 42 disposed between the fixing parts 41 for receiving the tilt yoke 23, a frame 40 provided at a rear surface with a board 43 for electrically connecting the wire suspensions 30 to each other, and a tilt coil 50 disposed around the yoke insertion member 42 to generate electromagnetic force around the tilt magnets 15.
The tilt magnets 15 are disposed having a polarity opposite to each other, opposing the tilt coil and the lens holder 10.
The lens holder 10 is provided at a rear surface with fitting grooves 16 for receiving the tilt magnets 15. The fitting grooves 16 are formed on left and right sides based on a central line of the lens holder 10 on which the object lens 11 is installed.
Instead of forming the fitting grooves 16, adhesive may be used to attach the tilt magnets 15 on the rear surface of the lens holder 10.
Alternatively, the tilt magnets 15 may be further provided on a front surface of the lens holder 10. In this case, plural tilt magnets 15 correspond to a single coil 50 in a state where the tilt yoke 23 is formed on a yoke plate 20.
The operation of the above described optical pick-up actuator of the related art will be described hereinafter.
The focusing and tracking motions of the optical pick-up actuator are realized by electromagnetic force generated by the magnet 21 attached on the yoke 22 and the focusing and tracking coils 13 and 12 of the lens holder 10, thereby reading and writing data.
The tilt motion of the actuator for eliminating the tilt aberration caused by the high RPM of the optical disk is realized by electromagnetic force generated by the tilt magnets 15 installed on the lens holder 10 and the tilt coils 50 installed on the tilt yoke 23 to tilt the lens holder 10.
That is, the tilt magnets 15 installed on the left and right sides of the lens holder 10 holding the object lens 11 are, as shown in FIG. 4, disposed having an polarity opposite to each other to perform the tilt motion using electromagnetic force generated by the tilt coil 50 disposed facing the tilt magnets 15.
In FIG. 4, the reference characters B, i and F indicates electromagnetic field, current, and Lorentz force. The Lorentz force is generated by interaction of the electromagnetic field B and the current i. The arrows beside the characters in the drawing indicate directions of the electromagnetic field, the current, and the Lorentz force.
That is, since the current i flows in a predetermined direction and the tilt magnets 15 have an opposite polarity to each other, a couple is applied to the lens holder 10 on which the object lens 11 is installed. Since the couple corresponds to moment applied in a direction of an X-axis, the lens holder tilts in a seesaw motion about the X-axis.
As the tilt magnets 15 and the tilt coil 50 are disposed spacing away form the focusing and tracking coils 13 and 12, such a tilt motion can be realized independent form the focusing and tracking motions.
By maintaining a predetermined air gap dx between the tilt magnets 15 and the tilt coil 50, magnetic flux between the tilt magnets 15 and the coil 50 is not varied in the course of performing the tracking motion.
Therefore, when compared with a hybrid-type optical pick-up actuator of related art, the 3-axis motion type optical pick-up actuator of related art has advantage of increasing an output constant in a direction of the tilting motion because the air gap dx between the tilt magnets 15 and the tilt coil 50 is set to have minimal distance when the tilt motion is realized, not affecting the tracking and focusing motions in a state where the electromagnetic force is generated by the tilt magnets 15 and the tilt coil 50.
As described above, the above-described 3-axis motion type optical pick-up actuator performs the 3-axis motion (focusing, tracking, and tilting motions) by providing the plural tilt-magnets 15 formed on the lens holder 10 and the tilt coil 50 independently installed in the yoke insertion member 42 between the fixing parts 41 of the frame 40.
However, the tilt magnets 15 separately installed on a rear surface of the lens holder 10 cause a size and weight of the lens holder to be increased, deteriorating the productivity and sensitivity.
Also, additional insertion and supporting structures for a magnetic circuit performing a radial tilt must be provided.
Further, since each of the wire suspensions is comprised of four numbers of wire springs and two number of coil springs to perform the 3-axis motion, complicating the structure and deteriorating the productivity.
Further, since an axis of the radial tilt is located on a rear side of the lens, an offset in a direction of a Z-axis is incurred.
Further, the two magnets 21 for focusing motion have the same magnetic flux density, this causing a pitching due to leakage flux.